[PC3] Add AbstractButton & Pane, make ToolBar padding come from Plasma theme

This patch introduces PC3 AbstractButton and Pane. It also makes ToolBar better at using the Plasma style. Because of trouble with correctly styling controls not based on PC3 in the past (e.g., Kirigami and plain QQC2), I thought it would be a good idea to fill out more of the PC3 QQC2 style. They currently aren't very different from stock QQC2 versions of those controls, but at least we have a bit more control.