Minor changes to the user interface
Add `symoroui/labels.py`
Rename files and Fix module import

Rename following files :
    symoroui/ui_definition.py -> symoroui/definiton.py
    symoroui/ui_geometry.py -> symoroui/geometry.py
    symoroui/ui_kinematics.py -> symoroui/kinematics.py
Fix module import statements in `bin/symoro-bin.py` to reflect new file
names.
Add docstrings to `bin/symoro-bin.py`
Start replacing label strings with dict keys

Add label strings as dict values in `symoroui/labels.py`
In `bin/symoro-bin.py` start replacing label strings with dict keys.
Remove `pysymoro/main.py`
Modify menu bar with dict keys

Replace the label strings in menu bar items with dict keys.
Add required key, labels string pairs in `symoroui/labels.py`.