hoping for better performance. On this occasion, add context menu
containing "Open" and "Choose folder" enabling drive changes on
Windows. Purge remains of QDirModel elsewhere.
Task-number: QTCREATORBUG-62

Task-number: BAUHAUS-120

Yet another quick and dirty hack while waiting for the real binding pass.

potentially reduces the number of file::exists() calls

partly by removing unnecessary calls, partly by providing minimalistic
reimplementations.
this gives a quite incredible performance boost ...

this gives a rather impressive speed boost.
if qmake wasn't trying to be so clever, we could get even more
out of this.

qmake doesn't, either

that way we save the creation of a temporary stringlist

now that we know the size of the entire file, we can use a single fixed
buffer instead of growing one on-demand - in the case where we just read
the file, we can even re-use the file buffer itself.
this should improve performance for files with very many line
continuations, but the practical impact drowns in the noise.
anyway, the code is nicer, as we don't abuse qstring that obviously any
more.

again several percent faster ...

in release mode they expand to nothing ... not so in debug mode.

- nest conditionals properly to avoid unnecessary tests
- avoid usage of QString::append(QChar) to save allocations

the latter will do it anyway.
this eradicates the performance penalty from the previous commit at its
root.

creating tons of items is a tad allocation-intensive and thus slow.

this is probably slower for often-included files, as now the splitting
is done in every evaluation pass.

now that the tree will not be written out any more, we can save some
cpu and memory by not recording the information.

that way the file formatting is better preserved.

just parsing it (once ...) ought to be sufficient. we don't need to
validate the syntax beyond that anyway.

Activate the function-like completion only when the character at the left of the text cursor is a `('.

Just a temporary hack to get completion for user-defined components. It should go away as soon as we have a decent binding pass.