Fix toggle playlist buttons

Reason for the change

Should probably be cherry-picked into the new stable branch

1. We were setting the drawer handle's visibility in two non-mutually-exclusive states. This meant the visibility of the drawer handle was dependent on the path you took to reach a state. I.e. make the window narrow and then enter party mode => handle is invisible; enter party mode and then make the window narrow => handle is visible
2. Kirigami now hides the mobile footer bar if the handleVisible is false. We were setting handle.visible: false and handleVisible: true which meant the mobile settings page actions were not visible.
3. When starting Elisa in narrow mode the drawer was open and then would shut immediately. Now it starts off closed.
4. The mobile show playlist button in grid/list views was broken. This was because contentView was referring the the contentView property of AbstractDataView, not the main ContentView.

Test plan

Test mobile page actions:

• Launch Elisa in mobile mode
• Make the window size narrow
• Open settings page or metadata form
• Previously: page actions were not visible
• Now: page actions are visible

Test party mode 1:

• Launch Elisa in desktop mode
• Enter party mode and then reduce window width to minimum
• Previously: playlist drawer button was visible
• Now: playlist drawer is visible (same behaviour)

Test party mode 2:

• Launch Elisa in desktop mode
• Reduce window width to minimum and then enter party mode
• Previously: playlist drawer button not visible
• Now: playlist drawer is visible

Test narrow mode:

• Launch Elisa and reduce width until playlist sidebar disappears
• Close and launch Elisa again
• Previously: Drawer started open and then closes immediately
• Now: Drawer starts closed