Move data type of axes limits and mouse plot pos from `float` to `double`

[SamuMazzi]

Description: Lately this topic has been discussed more than usual (like here) so I decided to make public what I've done some time ago. It's a simple and probably naive solution, so I'm open to improvements.

This solution actually closes #1847 (I've tested the example) and hopefully it should close #2067, #386 but on the other side I think it's still not enough precision for this #2157

Concerning areas: I've also read that @v-ein has (rightly) made the point of performance, but it shouldn't really be a problem (as far as I read, both from @axeldavy and the internet) also because double type has been already used in a while by ImPlot.

Notes: I've also changed the data structures from ImVec2 to ImPlotRange, I hope it's not a problem.