Replaces the following deprecated Gtk classes throughout the codebase by their non-deprecated counterparts. Style, HBox, VBox, HPaned, VPaned, HScrollbar, VScrollbar, VSeparator, HSeparator, Stock, Table, VScale and HScale.
Collection of CSS and GUI improvements, cleanup of main CSS theme, introduction of highlight for enabled module.
Primary change is the modification of the module titles to better distinguish modules that are on or off. The text of a disabled module is now less bright, the corresponding icon is a slightly smaller version of the power icon. When activating the module, the text turns brighter and the icon slightly bigger and also brighter. Thanks to @TechXavAL for the icon work.
Secondary changes are related to the margins around several GUI elements, padding within elements and some accompanying borders. These changes hopefully make it easier to distinguish the various (sometimes crowded) GUI elements of modules. The panels have gotten a slightly 'outset' look.
Nested panels have a changed look where deeper nesting increases the background brightness slightly, instead of darkening it (old behaviour). This is done without a strong decrease in contrast.
The old theme is available as a legacy option. Due to hardcoded GUI changes needed for the new theme, the legacy version is not a 100% exact replicate. The @TooWaBoo theme's may also be slightly affected.
This turns `IdleRegister::add()` into a template function to make the
provided function pointers type safe. It also adds a `delete_data`
parameter to manage the provided data pointer even in `destroy()`.
Improvement to the Batch Queue:
- Start Queue/Stop Queue checkboxes replaced with one swith.
- It is now immediately clear whether the queue is running or not.
- The queue is visually stopped once it is empty.
- String keys fixed to use BATCHQUEUE_ prefix instead of FILEBROWSER_.
For issue #3483, the old behavior is restored, i.e. the minimum width of
the ProfilePanel combobox is the maximum width of its content.
Style updated for the Exif and IPTC sub-tabs
Most conflicts seemed to be simple enough. There were a lot of `append_text` to
`append` conversions for `Gtk::ComboBoxText`. The `PopUpCommon` class also saw
a lot of changes with non-trivial conflict resolutions.
