Rename function createSection() into createMainContent()

The function’s name was misleading for its similarity to an html
section element. In fact, the function creates the main content within
a tab.
First attempt to get an option dialogand images into the share tab
Correct markup for better semantics in tab headline
add social modules

`social-network` defines everything necessary to interact with a
social network. That is, sharing episodes, shows and chapters and linking
to a profile.

`social-networks` lists all networks with their options available to the
player.

`social-button-list` groups a list of buttons to either share or link
to the respective profile.
use social module in info and share tab (WIP)
WIP shareoptions working!
Create a wrapping element for center positioning
of share buttons in share.js
Add color list for social service icons
Add new social services to service list
Create headline and input for sharelink in social link footer
Delete unused style for hr tag
Refactor color variables

- New names for default and contrast background variables were
necessary because of growing complexity of color schemes
Define variables for default padding and margin

and refactor all ocurrences.
Style share tab
Set new class for inverted colors in icon-fonts

Styling for squared share buttons with colored outside color and white
icon inside
Share Links: Set and display share link in input

- Set and display the default share-link of the episode in the input in
share footer
- Display the user-selected share-link in the input
- Styles for input, text and service buttons
Add missing color attribute for class 'selected'
Add service 'tumblr' to share module
Merge remote-tracking branch 'upstream/2.1.x' into new-share-tab

A refactoring caused issues with old playbutton styles.

Conflicts:
	src/sass/_buttons.scss
	src/sass/_mixins.scss
	src/sass/_titlebar.scss
Merge pull request #189 from podlove/new-share-tab

New share tab