gedit: Make better use of gtksourceview syntax

Rewrite the gtksourceview highlighting file to make better use
of available syntax options. This allows us to separate various
contexts, and means we don't have to keep track of all conky
keywords.

Overview of changes:

* Use separate contexts for config block and "lit string".
  * (lit string is the lua lit string [[ .. ]], we assume that
    this is used to contain text sections or template blocks)
* Use "sub-string" highlighting to highlight parts of identifiers
  separately.
* Added conky.text comments (from '#' to EOL).
* Remove lists of keywords and instead rely on syntax implications.
  * Uses separate contexts for config 'key=val' and text '${var}'s.
* Match escape sequences more precisely.
  * Lua and conky escape sequences matched separately; lua in normal
    strings and conky in literal strings. See escapes note.
* Transition from camelCase to hyphen-separated names
* Reword README to reflect changes

Escapes note:

We also match conky template escape sequences when in lua literal
strings as it would be too messy to separate `conky.text` and
`templateN = ` literal strings.

We currently don't match time format escape sequences as this would
require knowing which text variable we are in. This is only possible
if we duplicate the `bracket-var` context (less painful than
duplicating the `lua-literal-string` context).

Signed-off-by: Matt Sturgeon <matt@sturgeon.me.uk>

extras/gedit/README.md

@@ -1,16 +1,14 @@
 # Gedit syntax highlighting
 
-Note this is purely a highlighting **NOT** a syntax validator, the highlighting may not be 100% correct and not all arguments or keywords may be recognised properly. Feel free to change a regex if you have got a better one.
+Note: this highlights based on syntax and does **NOT** attempt to validate arguments or keywords. The syntax highlighting is unlikely to be 100% accurate and is open to improvement.
 
-The syntax highlighting will automatically be applied to all files with `conky` in their name. eg. `my_config.conky` (unfortunately it also triggers for the conky.lang file itself, you should set it to XML manually)
+The syntax highlighting will automatically be applied to all files with `conky` in their name. eg. `my_config.conky` (unfortunately it also triggers for the `conky.lang` file itself, you should set it to XML manually)
 
- *  [Gedit Syntax Highlight documentation](https://developer.gnome.org/gtksourceview/stable/lang-reference.html)
+ * [`gtksourceview` Syntax Highlight documentation][1]
  * [Regex Tutorial](http://www.rexegg.com/)
  * [Regex Testing](https://regex101.com/)
 
-Developers: if you want to add a new keyword just add it (in order) to the list. In `keywordsConfig` for config keywords and in `keywordsText` for variables. If it has a number at the end, eg `color1` just use `color[0-9]{1}` if the number is mendatory or `color[0-9]?` if it is not.
-
-If you want to add your own group of argument keywords you can look at the current definitions and copy them, should be self explainatory.
+Developers: The main context (`id="conkyrc"`) is where gedit begins. This main context then references other sub-contexts. Each context can apply styles to itself, sub-strings from its regexs, or its contents (in the case of `<start><end>` "container" contexts). If you are ever confused by something, try searching for XML attributes in the [`gtksourceview` docs][1]. If you find a particuarly complex regex, try using the Regex Tester linked above, and bear in mind that `gtksourceview` adds some extra regex syntax (i.e. `\%[ ... ]` and `\%{ ... }`).
 
 ***
 
@@ -29,3 +27,5 @@ If you want to add your own group of argument keywords you can look at the curre
 ` /usr/share/gtksourceview-3.0/language-specs/conky.lang`
  or (for single user)
 ` ~/.local/share/gtksourceview-3.0/language-specs/conky.lang`
+
+[1]: https://developer.gnome.org/gtksourceview/stable/lang-reference.html