[Feature] Custom theme for TUI

[kxxt]

Currently TUI is already theme-able at compile time. It would be nice to theme it at runtime by loading configuration files(TOML should be fine). And I want to make several themes.

Note that themes can also be installed system-wide.

[kxxt]

For now the theme is a global static variable. To implement custom themes, it need to be loaded from main func.

Unfortunately this would require static mut variable. But anyway I don't want to pass a reference to theme around in the code base.

[kxxt]

Themes should be loaded from ProjectDirs::data_dir()/themes first (user level themes) and if not found then path_to_tracexec_binary/../share/tracexec/themes.