Allow the user to set a custom table_view header view.
Adds header_will_display method to PM::TableScreen

If defined in your subclass, it will call:

header_will_display, header_will_display(view) or header_will_display(view, section)

This is so that you can modofy the view of a header before it is displayed.
Fixes issue #572
table view auto size row height functionality
Merge pull request #556 from clearsightstudio/tableview-headerview

Allow the user to set a custom table_view header view.
Merge pull request #563 from clearsightstudio/status_bar_style

Allow screens to choose their status bar style.
Merge pull request #567 from clearsightstudio/feature/views

Added on_load and on_styled hooks for UIViews
Merge pull request #569 from clearsightstudio/refresh_scroll_down

Scroll the table to show the refresh control unless the user initiates the refresh manually.
Fix conflicts, bring up to date with master
Merge pull request #570 from clearsightstudio/reorderable

Reorderable table screen cells
Resolved conflicts
Merge pull request #573 from clearsightstudio/bugfix/issue_572

Fixes issue #572
Fixed conflicts.
Merge pull request #574 from clearsightstudio/feature/auto-size-table-cell

Table view auto size row height functionality
Merge branch 'master' into blank_toolbar
Refactored a little
Merge pull request #577 from clearsightstudio/blank_toolbar

Auto-hide the toolbar when passed nil or false.
Version 2.2.0
Fix merge conflict & merge branch 'master' into header_will_display

* master: (21 commits)
  Version 2.2.0
  Refactored a little
  Auto-hide the toolbar when passed nil or false.
  table view auto size row height functionality
  Fixes issue #572
  Add longpressable example table screen to demo app.
  Implement @silasjmatson's idea about moving move_cell into promotion_table_data
  Allow the user to restrict the cell's movement within a section.
  Fix failing tests.
  Moar specs! 🙀
  Change edit? to edit_mode?
  Make cell_moved on_cell_moved to conform to on_cell_deleted.
  Fix true|false bug.
  Tests for moveable cells.
  Initial moveable cells functionality.
  Scroll the table to show the refresh control unless the user initiates the refresh manually.
  Version 2.1.0 + README update
  Added on_load and on_styled hooks for UIViews and on_reuse for UITableViewCells
  Make second param a hash instead of just the animation.
  Allow screens to choose their status bar style.
  ...

Conflicts:
	app/test_screens/test_table_screen.rb
Change will_display_header to header_will_display