Lots of small changes to make the code more readable

Reason for the change

Lots of small things

Test plan

Test 1

1. Open party mode

• Before: qrc:/qml/PlayListEntry.qml:391:13: Unable to assign [undefined] to bool
• After: no error

Test 2

(To check that commit a13b8ee7 didn't change up the playlist focus)

1. Open multiple items in the playlist (enough for scrollbar to appear)
2. Click the top item
3. Scroll until the top item is hidden
4. Press shift + tab, then tab

• Before: top-most visible item is selected
• After: same as before

Test 3:

1. Scroll over duration slider & volume slider

• Before: Signal parameter injection warning
• After: No warning

Test 4:

1. Open in mobile mode QT_QUICK_CONTROLS_MOBILE=1 elisa
2. Click the bottom bar to open the Now Playing screen
3. Click somewhere on the Now Playing screen

• Before: Signal parameter injection warning
• After: No warning