Update QDarkStyleSheet to 2.8 and apply workaround for combo-boxes

This fixes the problem with tab-bar scroll buttons reported in sqlitebrowser#1968

This version has a problem with combo-boxes with icons, so the recommended
workaround is implemented for the language combo-box in Preferences and the
Table combo-box in Browse Data.

ColinDuquesnoy/QDarkStyleSheet#169

Update version number to 3.11.2

The rebuilt macOS installer (v2) was generated from this tag.

Update version number to 3.11.1

We're using Qt 5.11.3 for the Win64 builds

The DB4S 3.11.0-beta3 Win32/64 & macOS binaries were created from

this commit.

The DB4S 3.11.0-beta2 Win32/64 & macOS binaries were created from thi…

…s commit.

The DB4S 3.11.0-beta1 Win32/64 & macOS binaries were

created from this commit.

This commit is what the DB4S 3.11.0-alpha1 Win32/64 & macOS binaries …

…were created from.

Update version numbers to 3.10.1