Add Jinja2 test for buttons and a macro for form actions

Also changed hidden loops in bootstrap_wtf to use
hidden as a filtering test.
Have all WTForms imports specify a module

Trying to make it easier to understand which class
comes from where.
Add support for placeholder on types that support it

Also add an example of it in the sample.
Fix the HTML5 widgets based on Input

input != input_type
Factor out error-or-help message to a macro

Will be used in upcoming change.
Support other form layouts on quick_form

Also add an example of an inline form.
Add required in widgets, add more HTML5 widgets

Also removed now-obsolete line in inline_field macro.
Organize widgets and fields into separate modules

There was also some changes, mainly in proper
separation of classes/mixins.