Added TabStackedWidget + ComboTabBar

Set of patches:
Introduce TabStackedWidget+ComboTabbar
[Windows] Fix enabling blur background in fullscreen
[ComboTabBar] Fix some issues with pinned tabs
[ComboTabBar] Don't use pointer to integer,
Implement TabStackedWidget::keyPressEvent() and fix changing tab by wheel event
Fix animation delay of dragged tab's buttons.
[ComboTabBar] Add themable scrollButtons
TabBar: Edit sizes of tabs. Minimal tab width is now 100
[ComboTabBar] Fixed weird scrolling on resizing.
[ComboTabBar] Workaround for setting tabbbar height with Oxygen
[ComboTabBar] m_heightIsDirty is not really needed
[ComboTabBar] Tweaked appearance of buttons when tabs overflows.

Also disabled painting shadow gradient indicationg that tabbar is not
scrolled at the end.
[Coding style] It should be only one if statement...
Added Tab scrolling feature when tabs don't fit in tabbar.

Closes #432