Time Window selection

### Description

Before starting a typing session, we should allow the user to change the default timer value (currently it's fixed to 30sec) similar to what other platforms like `monkeytype` provides.

User can select one of the following three values:
* 15 seconds
* 30 seconds
* 1 minute

For now, this can be done on the starting screen itself and later moved to a session config screen.