Misc control improvements - S1

[FeodorFitsner]

Blog announcement: https://flet.dev/blog/gradients-button-textfield-style

What's new

• New Container properties:
  • gradient
  • on_long_press
• New ElevatedButton, OutlinedButton, TextButton properties:
  • style
  • on_long_press
• New IconButton properties:
  • selected
  • selected_icon
  • selected_icon_color
  • style
• New ListTile properties:
  • on_long_press
• New Theme properties:
  • visual_density: standard (default), compact, comfortable, adaptivePlatformDensity.
• hidden value for Column, Page, View, Row and View controls - to enable scrolling but hide a scrollbar.
• New TextField and Dropdown properties:
  • text_size
  • color
  • bgcolor
  • border_radius
  • border_width
  • border_color
  • focused_color
  • focused_bgcolor
  • focused_border_width
  • focused_border_color
• New TextField properties:
  • max_length
  • capitalization - none (default), characters, words, sentences.
  • cursor_color
  • selection_color

Bug fixes

• Fixed page.width and page.height are 0 on the first page load #138
• Fixed Allow partial theme updates #77
• Fixed Container on_click callback breaks containers without explicit size #140
• Fixed Assertion "lookup_hash_table" fails on ToDo App example #30

Other changes

• Flutter SDK switched to dev channel.
• Fixed Changing app host/IP binding #98