Make methods on `Time`, `Timer`, and `Stopwatch` more consistent

[rparrett]

Bevy version

0.14 / main

What you did

Noticed some small inconsistencies between Time, Timer and Stopwatch while refactoring.

What went wrong

This may not be totally comprehensive.

Time	Timer	Stopwatch
elapsed_seconds	elapsed_secs	elapsed_secs
elapsed_seconds_f64	DNE	elapsed_secs_f64
is_paused	paused	paused

At a glance, it seems like we should

• Add _f64 stuff to Timer
• Rename *_seconds to *_secs or vice versa
• Rename paused to is_paused or vice versa

Additional information

When deciding on renames there may be other methods to look at (animation, audio, whatever) with similarly-named methods. e.g. AudioSink::is_paused, Duration::from_secs.