DOC: guide2 minor edits to kivy basics and kvlang sections
Doc: fix file name
add reference to metrics module in guide2 and to screen module
formatting fix in guide2/widgets
add png version of Kivy_App_Life_Cycle
clarify a little the metrics paragraph in guide2/widgets
small fixes in guide2/lang
doc: fix the app lifecycle image: bring back the svg into the source/images, and use the png version.
Add more explicit note about naming the kv file.
Doc:TextInput: add Note about managing text selection in on_focus
UIX:TextInput fix first line duplication on `enter`
DOC:Guide2 add to customize section  in Kivybasics
logger: fix memory leak when log_enable is 0. closes #886
label: fix text rendering with empty markup. closes #883
uix:TextInput: avoid situations that could lead to cursor_row being off
screen. closes #885