Reduce input detection CPU usage

[Abestanis]

This reduces the CPU consumption of the FieldSet generation and speeds up the progress. I measured the runtime of the detectFields function:

Test page	Runtime before the patch	Runtime with the patch	Approximated difference
A simple page without password inputs	0.319 ± 0.074 ms	0.136 ± 0.033 ms	-57% (~2.34x faster)
A complex page with one password input (virustotal.com)	0.322 ± 0.086 ms	0.133 ± 0.032 ms	-59% (~2.42x faster)

The main improvement is not iterating over all the inputs, but rather searching for previous input elements starting from the password inputs.