misc: fix image button svg rendering and cleanup button components

- provides a base component for rendering scaled svg images
- simplifies the normal/hovered/pressed state transitions
- uses color overlay to factor out image source changes for
  checked states
- fixes sizing inconsistencies and visual bugs
- removes manual background color changes and minimizes the use
  of MouseArea components
- removes the unused HoverableGradienButton component

Gitlab: #14
Change-Id: I06357d3cc6d8d04d9d1f062753dc39223a64d4f6