Add highlight and show how to use some features

[minervapanda]

I could not upload the whole video.
This pr addresses :
[x] Swipe to remove the chatbox
[x] Switching streams from the menu
[x] Private chat drawer
[x] Long press on message for more options

Solves : Highlight and show how to use some features of the app which are not easily visible - #161

[smarx]

Automated message from Dropbox CLA bot

@minervapanda, it looks like you've already signed the Dropbox CLA. Thanks!

[kunall17]

Thankyou for working on this!

• Rather than creating activities which shows static images, you could easily make multiple showcase views like it is done here https://github.com/amlcurran/ShowcaseView/blob/master/sample/src/main/java/com/github/amlcurran/showcaseview/sample/MemoryManagementTesting.java (Got this one from this issue How to use views in sequence on after another ?plz help amlcurran/ShowcaseView#325) in the ZulipActivity.java file
  
  And while switching showcases you could programitically open/close the drawers etc
• Add a help option in the overFlow menu which would show these highlights
• Tweak with background and foreground text Color as the current scheme makes it hard to read the text. Maybe decrease the opacity of the background if possible!

[kunall17]

And also remove the commit a475f45 from this PR, as this is not related to this feature!

[minervapanda]

Hi @kunall17 , I added the help option to the menu and similar way of implementation is in ChatBoxFeature java file. Give a look. We need to show different layouts for different demonstration for other features.
Also removed the unwanted commits

[brainwane]

@minervapanda could you add a little more detail to the commit message (customs explained https://zulip.readthedocs.io/en/latest/version-control.html#commit-messages ) to list the features that get demonstrated in this new showcase?

[brainwane]

I think we don't capitalize Left here.

[brainwane]

I think we usually use "Tap" here instead of "Press".

[niftynei]

Hey @minervapanda, have you had a chance to look over the suggestions from Sumana and Kunal yet?

[yadav-rahul]

@minervapanda Due to recent merges in master, merge conflicts have been created. Please try to resolve them.

[saketkumar]

@yadav-rahul This PR is no longer valid.
@timabbott @kunall17 I think this should be closed.

[zulipbot]

@minervapanda, your pull request has developed a merge conflict! Please review the most recent commit (f791888) for conflicting changes and resolve your pull request's merge conflicts.