This commit solves #212, solves #219, solves #221, solves 222, solves #220, solves #225, solves #226, solves #227.

#212
- When speakers for the recording are added, UI is changed to use checkboxes and okButton.
- To support the UI, RecordingSpeakersActivity, RecordingSpeakersArrayAdpater are created.
- These 2 classes replace the use of SpeakersAcitivity, SpeakerArrayAdapter in the RecordingMetadataAcitivity.

#219
- When custom language is added to a language list, it is checked by default.

#221
- Red-cross button is now deleted from the RecordingMetadataAcitivity

#222
- Language selection in the recording is not reset after new speaker is added in the RecordingMetadataAcitivity.

#220
- AddSpeakerAcitivity is now divided into 4 stages(name, language, photo, final confirmation)
- Consistent UI when back-button is pressed. 
(If the user inputs nothing, smoothly go back 1 stage, otherwise pop up the dialog box with a customized message)
- Consistent UI for each stage(If the user inputs nothing, okbutton is greyed out)
- Previous inputs are kept and shown in the next stage
- The order of speakers is change (new(top) -> old(bottom))

#225
- Before/After save icon is pressed, if the duration > 250msec, prompt the warning dialogue box.

#226 : language string in AddSpeakerActivity2 is changed
#227 : addSpeakerButton, recordSaveButton is now at bottom

#ETC
- SpeakerLanguagesArrayAdapter, RecordingLanguagesArrayAdapter are deleted and replaced by LanguagesArrayAdapter.
- Consistent UI when speakers are added to recording's metadata.
(If the user selects no speaker, no warning-dialog-box and greyed-out-okButton)