Introducing `AdminPagePreBuild` delegate

This delegate is needed in some cases in order for extensions to do things before the content pages (JSON/XML/...) are instanciated. Using `AdminPagePreGenerate` was not working in some cases because the `view()` method (where most of the code for content page goes) is called by the `build()` method called in `__buildPage`.
Move all requires to a single autoload file
Implement classmap autoloader
Move the autoload into the index instead of bundle
Don't accidentally load the Administration class during initialisation
Add composer autoloader
Fix Installer's autoloader
Consistently refer to Datasource class
Add in the interface autoload classes
Couple of class references updated for case sensitivity
Bump version to 2.6.0-alpha.1
Add fetchFilterableOperators to the Field class, RE: #2073
Handle existing filter values and display the correct help text
Make the XSRF token live for as long as the session, remove some old array handling code. Closes #2174
Don't order empty tables

Fixes #2239
Fix condensed settings

Closes #2240
Dim hidden sections

Closes #2105
Conditionally highlight password confirmation

Closes #2065
Fixed #2235
Publish filtering count

Fixes #2129