Validate the Z value input

[ali1234]

Checks that the Z value and row and column are numbers and enables the save button as appropriate. Automatically rounds the input field to the displayed precision. The precision may be adjusted, perhaps with a configuration option?

Also tweaks the HTML: Changes the default step from 0.025 to 0.01 (this seems to be the default precision in Marlin) and sets autocomplete="false" on the save button, to prevent Firefox from remembering the state across reloads, potentially leaving the button enabled when it should not be.

Fixes #18