Add manual GUI size setting to override desktop environment DPI

[GRbit]

• [x] I checked the issue tracker for similar issues
• [x] I checked the changelog if the issue is already resolved
• [x] I tried the latest Clementine build from here

System information

• Operating System: MX Linux XFCE
• Clementine version: 1.4.0rc1-875-g351a5e254

Expected behaviour / actual behaviour

When I set up DPI for my desktop environment, changing it from 96 to 144, Clementine doesn't react until I come to 144. On 143 it's fair small, but on 144 it's just too big for 2k monitor. It would be great to be able to control GUI scaling/size from Clementine settings. It would be even better it it would be possible to set some scaling factor in between the current ones.

The main idea is to have a field to set up DPI manually for Clementine (and a ticker to use system value).