CppEditor: fix "follow symbol under cursor" for operators

Like for functions we can call attemptFuncDeclDef for operators. The
check for function names has been enhanced to take operators into
account.
For the switch from definition to declaration
SymbolFinder::findMatchingDeclaration has been enriched with operator
knowledge.

Task-number: QTCREATORBUG-7485

Change-Id: I29eebee337e26c8bf67dc8b4a15c43883045589d
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
BuildConfigurationFactory: Introduce priorities

Introduce priorities for build configuration factories. This way
plugins can register specialized build configuration factories, that
e.g. can provide additional build steps.

A negative priority signifies that a factory is not prepared to
handle a request, the default build configuration factory shipped by
the build system plugin will report a priority of 0. Add 100 to that
for each specialization you add (e.g. a remote linux buildconfiguration
factory would report 100, a specialization of that for mer will
should report 200, etc.).

Change-Id: I141a7a5a79166afdb7657d46eb7e86bd18d3abf6
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Michal Klocek <michal.klocek@digia.com>