Splits: small UI fixes

• Change expanded mode button tooltip

  Toggles use descriptions/names of an active state only. New string is consistent with KWin, which uses "Maximize" for maximization toggle action.

• Fix typo in tooltip context

• Use default system font for terminal header titles

  This is not only style change, but also a bugfix. Header and title font was unspecified, so terminal monospace font was used in most cases. However, when new split with different orientation was created, the font of splitted terminal changed to the system font.

  Steps to reproduce:

  • Split vertically - both titles use monospace font
  • Split horizontally - splitted header's font changes to system default