[Yakuake] Improve customization

Summary:
This patch increases Yakuake customization possibilities without break existing skins.
The new features are:

• new tab button can be positionned at the end of tabs
• selected tab text can be not bold
• titlebar's title can be centered
• titlebar's icons can be anchored at left
• a compact mode that superposes title and tabbar

Here examples of what can be achieved with these changes:

Test Plan:
Install skins:

Reviewers: hein, Yakuake

Reviewed By: hein

Tags: Yakuake

Differential Revision: https://phabricator.kde.org/D19835