Focusing a markdown cell should enable it for change

[josevalim]

Today I need to explicitly press the Edit icon on the top right.

[jonatanklosko]

This is also intentional, clicking on a markdown cell just focuses it (the user may want to copy some text or see cell options). The editor shows once you enter insert mode (either i or the pencil in this case). This keeps the clear distinction between insert and navigation mode. Pretty sure that being too eager about entering insert mode for markdown cells is not gonna feel nice, but maybe it's just me ^^