GUI improvements and new GUI style menu

Contains the following GUI improvements that are also discussed in #674 (closed):

• Added a new input component InputField that replaces InputText, InputCheckboxes and InputSelect by combining their functionality into a single component.
• Aggregations are no longer pre-loaded if they are not needed immediately. Aggregations will be loaded on-demand e.g. when opening a menu or displaying a statistic.
• Reduced the size of terms aggregations. Previously a greedy default of 200 items was used, but now only the amount that is necessary for visualization is loaded. The amount is determined from the maximum size requested by any visible component. InputField has controls for increasing the requested size (it currently uses a fairly naive approach of just increasing the aggregation size, scrolling could be possible if it does not interfere with exclude_from_search).
• Statistics are now normalized to the aggregation maximum.
• The search bar has been restyled to be more recognizable. I moved the search icon to the beginning (like e.g. in GMail search) and made the placeholder text always visible. We could also try using some colour to highlight it more, but blue buttons are not consistent with the rest of the UI and blue outline is not compatible with the focused state.
• Modified the search column defaults to be less method-oriented.
• Fixed issue with exclude_from_search being enabled even though no user interaction had been done to the filters.
• Removed the undefined statistics in the visibility-filter.
• Introduced a new style for the GUI:
  • Different icon and icon size for the button that enables/disables statistics for a filter.
  • Different text input style and size
  • Statistics scaling is now shown directly and not hidden inside a menu
• Added a new menu for playing around with the GUI style. This menu is hidden by default, but can be enabled by pressing Alt+G.