Extract IBus related GTK style (4.0.0)

[HollowMan6]

This PR intends to add functionality to extract IBus related GTK styles so that users or IBus theme designers can easily modify those styles to adapt to their own needs.

GTK+ CSS Overview

Problems to solve (In Priority Order):

• At present, it seems like that the styles in the original style sheet cannot be overwritten by specifying the #IBusCandidate name alone.

  目前似乎通过限定名称#IBusCandidate来单独指定出来并不可以覆盖原主题中的类样式。

• At present, there are many redundancies in the extracted styles, which can be further cleaned up.

  目前拆解出来的样式存在许多冗余，可以进一步清理。

• Extraction of the style sheets using like @import url("resource:///...") by extracting gresources files.

  通过解包gresources文件，实现对于类似于@import url("resource:///...")类的引用的样式表解析。

• Allow users to easily select a background picture for GTK themes and changing it's border radius.

  允许用户很容易地为GTK主题选择一个背景图片并更改其圆角半径。

• Handling resources use like pictures etc.

  处理对于资源的引用，如图片等。

• Merge multiple gtk.gresource files into one.

  将多重gtk.gresource文件合并为一个。

• Candidate Highlight seems not to be able to overwrite.

  候选词高亮色似乎无法被覆盖。

Signed-off-by: Hollow Man hollowman@hollowman.ml
测试

[HollowMan6]

Will merge now since most of the important issues have been resolved.

[HollowMan6]

Getting some ideas from ibus/ibus@bdc83bc , hopefully to resolve candidate Highlight seems not to be able to overwrite.

[HollowMan6]

Still can't find a way to resolve this.

[HollowMan6]

Move to #2 for discussion