card filter: add clear button and placeholder text

[tooomm]

What will change with this Pull Request?

• Adds clear button to card search text field
• Adds placeholder text to card search text field
• I rearranged my change from deck editor: add placeholder text to search #3000 to find a better line for it

Screenshots

It now looks like this:

• Before you start typing you see the placeholder text:
  
• During typing it disapears, but you'll see a clear button instead to delete all line at once:
  

[ZeldaZach]

1. Tabs->Spaces
2. Remove the #if guards for the clearButtonEnabled. We build exclusively on QT5 nowadays and we do not have to worry about that

[ZeldaZach]

We build above 5.0.2, so these guards should be removed at this time.

[tooomm]

I mirrored them from another place in the app.
Will remove both then 👍

[tooomm]

postponed, see #3005

1. is addressed // 2. is not yet possible, see #3005

[ZeldaZach]

Sounds good