Place the trojita icon in the right directory

The trojita icon is 64x64 but was being placed in
/usr/share/icons/48x48/* directory. resolved to store in
/usr/share/icons/64x64/*

BUG: 332640
REVIEW: 117115
Installed into system locations, trojita can't be started

This fixes not looking in standard data locations for the installed qml
files. To fix this it first checks if main.qml exists in the current
source tree, if not then we get a list of standard data locations and
try to find main.qml and throw qFatal if can't be found.

BUG: 332639
REVIEW: 117109