Add methods for finding a font with the right size

[codetent]

Needs to be checked with the mentioned Linux distribution if this solves the issue #10.

[Akuli]

I have read your changes.

• Instead of a tuple conversion in teek/_font.py, it should return to_tcl(self._font_description), where to_tcl is imported from _tcl_calls.py. This was a bug. It should have always called to_tcl on the font description before returning it, as is documented here:

If the value has a to_tcl() method, it’s called with no arguments. It should return a string that will be passed to Tcl.

Edit: I don't want to add runtime type checks to detect these problems because the code in _tcl_calls.py is very performance-critical.

• Aren't _check_font, _create_font and _create_named_font a bit unnecessary? I think using teek.Font.families()[0] instead of 'Helvetica' might be enough, because the size should behave nicely for a font that is actually installed on my system. This is true for at least every font on this system, as I have shown here.

Edit 2: The above sentence used to say "necessary", but I meant "unnecessary".

[codetent]

• I changed the tuple conversion to to_tcl
• I checked the size property for each font family installed on my system and discovered that the actual size can also vary if the font is available. Therefore, I replaced 'Helvetica' with a function call which returns a font family whose size is equal to the given one.

[Akuli]

Awesome :D