Fix CondDBBrowser runtime and drop Qt4

• add runtime dependency on libxkbcommon.so
• drop support for PyQt4
• reformatted browser code (with yapf)
• improved about dialog

• [2018-04-14 00:05] Validation started with lhcb-moore-test#26
• [2018-04-14 00:16] Validation started with lhcb-2018-patches#180

merged

mentioned in commit 59c5958c

mentioned in commit 3bbe875d

mentioned in merge request !1244 (merged)