Simpler way of installing python packages to created python virtualenv

Currently the created environment only puts python binary in there. There is no activate script that virtualenv would create. There isn't even pip binary.

And the new environment also does not have PySide installed by default so it is unusable anyway by default.

It is possible to install stuff there with path/to/venv/bin/python -m pip install ..., but how many users would actually know this.

Some improvements that come to mind:

• make Qt Creator set PATH to the bin folder automatically when using the new built-in terminal.
• create pip binary also to the virtual environment
• add activate script(s) so that it is easier to use from terminal
• add a box in the wizard to add packages to install
• add same thing also to Projects panel