Unify Tab Style + Update Undock Icon

[Takanu]

• Added MM_FlexibleTab references to all other theme files
• Added the base_type to TabBar to unify the look and to ensure adequate padding is used on either edge of the tab.
• Updated the undock icon to use a SVG file. The edited icon now aligns with the close icon and the triangle glyph is filled to prevent it from looking fuzzy at small UI scales.

(I hope this PR is formatted correctly, I'm still new to this. Let me know if anything needs changing).